Norwood, Glenn and Fontan
Introduction
Congenital cardiac defects can often be approached by asking three questions:
How many ventricles are actually capable of doing useful work?
Where is the blood flowing?
How can pulmonary and systemic circulation be separated effectively?
This provides another way of looking beyond the simple distinction between the “pink baby” and the “blue baby.”
In single-ventricle physiology, the central problem is that two effective pumping chambers cannot be established.
One ventricle must therefore initially support both the pulmonary and systemic circulations.
The long-term surgical strategy does not attempt to recreate a normal two-ventricle heart.
Instead, the circulation is progressively reconstructed so that the single functional ventricle is reserved predominantly for the systemic circulation, while venous blood is ultimately directed towards the lungs without the assistance of a dedicated subpulmonary ventricle. (PubMed Central (PMC))
This is the principle behind the staged pathway of:
Norwood → Glenn → Fontan
Body
What Does “Single Ventricle” Mean?
The term single ventricle does not necessarily mean that only one anatomical ventricle exists.
In many cases two ventricular chambers are present.
The problem is that only one can function effectively within a sustainable circulation.
This may occur because one ventricle is severely underdeveloped, because an atrioventricular valve is absent, because systemic or pulmonary outflow cannot be established adequately, or because the ventricular anatomy cannot safely be divided into two independent pumping systems.
Examples include:
- Hypoplastic Left Heart Syndrome;
- tricuspid atresia;
- double-inlet left ventricle;
- some unbalanced atrioventricular septal defects;
- some forms of double-outlet right ventricle;
- pulmonary atresia with severely hypoplastic ventricular anatomy;
- mitral atresia;
- and complex heterotaxy-associated heart disease. (PubMed Central (PMC))
The common feature is therefore not one particular defect.
It is the physiology.
The Problem With One Functional Ventricle
In the normal heart, the right ventricle pumps blood through the lungs and the left ventricle pumps blood around the body.
Each ventricle has one job.
In single-ventricle physiology, one chamber may initially have to deal with both circulations.
Oxygenated pulmonary venous blood and deoxygenated systemic venous blood frequently mix before entering the functional ventricle.
That ventricle then has to supply both:
pulmonary blood flow
and
systemic blood flow.
This is inefficient and creates substantial volume loading.
The surgical objective is therefore not merely to improve oxygen saturation.
It is gradually to remove the pulmonary circulation from the workload of the single ventricle.
The Principle of Staged Palliation
The eventual aim is a circulation in which:
Systemic venous blood → Lungs
and
Pulmonary venous blood → Single ventricle → Body
The remarkable feature of the final Fontan circulation is that venous blood reaches the lungs without being actively pumped there by a ventricle.
Achieving this in a newborn would generally be physiologically impossible.
Pulmonary vascular resistance is initially too high.
The circulation therefore has to be reconstructed in stages as the child grows and pulmonary resistance falls.
The exact pathway depends upon the underlying anatomy.
Not every child with single-ventricle physiology requires exactly the same Stage I operation, and not every single-ventricle patient undergoes a Norwood procedure. Where systemic outflow is obstructed, however, Norwood-type reconstruction is a principal Stage I strategy. (PubMed Central (PMC))
Stage I — The Norwood Procedure
Why is a Norwood Needed?
The Norwood operation is most strongly associated with Hypoplastic Left Heart Syndrome — HLHS.
In HLHS, the left-sided structures may be too small to sustain systemic circulation.
The left ventricle cannot function as the systemic pump.
Instead, the right ventricle must become the systemic ventricle.
This creates a major problem.
The right ventricle normally ejects into the pulmonary artery.
The Norwood operation therefore reconstructs the circulation so that right ventricular output can reach the aorta and systemic circulation.
The fundamental Stage I objectives are:
- create unobstructed systemic outflow;
- ensure unrestricted mixing at atrial level;
- and establish a controlled source of pulmonary blood flow. (PubMed Central (PMC))
Constructing the Neoaorta
In severe left-sided hypoplasia, the native ascending aorta may be far too small to carry systemic cardiac output.
The pulmonary root is therefore recruited into the systemic circulation.
The proximal pulmonary artery is connected to the diminutive native aorta and aortic arch.
This reconstructed outflow becomes the neoaorta.
The right ventricle can now eject:
Right ventricle → Pulmonary root / neoaorta → Systemic circulation
The right ventricle has effectively been converted into the systemic pumping chamber.
What Happens to Pulmonary Blood Flow?
Once the pulmonary root is recruited into systemic circulation, a new route is required to supply the lungs.
Traditionally this has been achieved with a modified Blalock–Thomas–Taussig shunt connecting a systemic artery to a pulmonary artery.
Another option is the Sano conduit, which connects the right ventricle directly to the pulmonary artery.
Both strategies aim to provide sufficient pulmonary blood flow without allowing so much flow that the systemic circulation is compromised. (cpr.heart.org)
This balance is critical.
Too little pulmonary blood flow causes profound cyanosis.
Too much pulmonary blood flow steals cardiac output from the systemic circulation.
Atrial Septectomy
The pulmonary veins return oxygenated blood to the left atrium.
In HLHS, that blood needs to reach the functional right-sided ventricle.
An unrestricted atrial communication is therefore essential.
The atrial septum is opened or removed to allow free mixing between the atria.
The circulation after Stage I therefore remains a parallel and mixed circulation.
The Norwood does not separate oxygenated and deoxygenated blood.
It creates a survivable circulation until Stage II can be performed.
The Interstage Period
The period between the Norwood operation and the Glenn is particularly vulnerable.
The circulation depends upon a delicate balance between pulmonary and systemic blood flow.
Changes in:
- pulmonary vascular resistance;
- systemic vascular resistance;
- shunt resistance;
- ventricular function;
- hydration;
- haemoglobin;
- and oxygen demand
can have major consequences.
This is why many centres use structured home-monitoring programmes during the Norwood-to-Glenn interstage period.
Single-ventricle patients remain among the more complex and resource-intensive groups within congenital cardiac care. (PubMed Central (PMC))
Stage II — The Bidirectional Glenn
The second major stage is the Bidirectional Glenn, usually undertaken during infancy after pulmonary vascular resistance has fallen sufficiently. Contemporary descriptions commonly place it around 3–6 months of age, although timing varies with the patient and centre. (PubMed Central (PMC))
The principle is fundamentally different from the Norwood.
Instead of using ventricular output to send all blood to the lungs, part of the venous circulation is connected directly to the pulmonary arteries.
What Happens During the Glenn?
The Superior Vena Cava — SVC carries systemic venous blood from the head, neck and upper body towards the heart.
During a bidirectional Glenn, the SVC is disconnected from the atrium and connected directly to the pulmonary artery.
Thus:
Head and upper body → SVC → Pulmonary arteries → Lungs
No ventricle pumps this blood into the pulmonary circulation.
It flows because venous pressure is higher than pressure within the pulmonary vascular bed.
The continuity between the branch pulmonary arteries is preserved so blood can flow to both lungs — hence bidirectional Glenn. (PubMed Central (PMC))
Why Does the Glenn Help?
Before the Glenn, the single ventricle effectively has to pump:
- systemic blood flow;
- plus pulmonary blood flow.
After the Glenn, much of the venous return from the upper body travels directly to the lungs.
The ventricular volume workload is therefore reduced.
This is sometimes described as volume unloading of the single ventricle.
The Glenn also begins the process of separating systemic and pulmonary circulation.
But it is not complete.
Venous blood from the lower body still returns through the Inferior Vena Cava — IVC to the heart and mixes with pulmonary venous blood.
The child therefore normally remains somewhat cyanosed.
The Classic Glenn and the Bidirectional Glenn
Historically, the original classic Glenn shunt connected the SVC to the right pulmonary artery while separating the right and left pulmonary arteries.
Only the right lung therefore received SVC blood.
Modern practice predominantly uses the bidirectional Glenn, in which the pulmonary arteries remain connected and SVC blood can reach both lungs. (PubMed Central (PMC))
This distinction is useful because the word “Glenn” is still often used rather loosely to describe both historical and contemporary procedures.
The Hemi-Fontan
An alternative Stage II operation is the hemi-Fontan.
Physiologically it performs a similar role to the bidirectional Glenn by directing superior vena caval blood into the pulmonary circulation.
Its surgical construction differs and is particularly useful in patients intended for a later lateral-tunnel Fontan completion.
For the purposes of basic physiology:
Bidirectional Glenn and hemi-Fontan both represent partial cavopulmonary connection.
Stage III — The Fontan
The Fontan represents the final planned stage of conventional single-ventricle palliation.
The principle is:
complete the connection of systemic venous blood directly to the pulmonary arteries.
After the Glenn, only the SVC blood bypasses the ventricle.
During Fontan completion, blood from the IVC is also directed into the pulmonary circulation.
The result is a total cavopulmonary connection.
Fontan completion is commonly undertaken in early childhood, often around 2–4 years, depending upon anatomy, haemodynamics and institutional practice. (PubMed Central (PMC))
The Fontan Circulation
Following Fontan completion:
SVC → Pulmonary arteries
and
IVC → Pulmonary arteries
Systemic venous blood therefore reaches the lungs without passing through a ventricle.
Oxygenated pulmonary venous blood returns to the heart.
The single functional ventricle then pumps:
Pulmonary venous blood → Single ventricle → Systemic circulation
The single ventricle is therefore finally relieved of the task of pumping blood through the lungs.
It becomes principally the systemic pump.
There is No Pulmonary Ventricle
This is perhaps the most important concept in understanding Fontan physiology.
A normal circulation has:
Right ventricle → Lungs
and
Left ventricle → Body
A Fontan circulation has:
Systemic veins → Lungs
and then
Single ventricle → Body
There is no subpulmonary ventricle.
Blood must travel through the pulmonary circulation because systemic venous pressure is sufficient to push it through a low-resistance pulmonary vascular bed.
That makes pulmonary vascular resistance critically important.
Even relatively modest increases in PVR can substantially impair Fontan circulation. (PubMed Central (PMC))
Types of Fontan
The Fontan operation has evolved considerably since its introduction in the early 1970s.
Atriopulmonary Fontan
Early Fontan operations connected the right atrium directly to the pulmonary artery.
These operations allowed many patients to survive but produced major long-term problems associated with a massively dilated, high-pressure right atrium and atrial arrhythmias.
They have largely been superseded by newer approaches. (PubMed Central (PMC))
Lateral Tunnel Fontan
A baffle is constructed within the right atrium to channel IVC blood towards the pulmonary artery.
Extracardiac Conduit Fontan
A tube graft outside the heart connects the IVC to the pulmonary arteries.
The extracardiac conduit is now a widely used form of Fontan completion. (cpr.heart.org)
Fontan Fenestration
Some Fontan operations incorporate a small deliberate communication between the Fontan pathway and the atrium.
This is known as a fenestration.
If Fontan pressure becomes excessive, some venous blood can pass through the fenestration into the heart.
This acts as a pressure-relief route.
The cost is that deoxygenated venous blood enters the systemic circulation.
The patient therefore accepts a degree of cyanosis in exchange for lower venous pressure and potentially improved cardiac output.
It is effectively a controlled pop-off valve.
What Makes a Good Fontan Circulation?
The Fontan depends upon remarkably favourable physiology.
Important requirements include:
- low pulmonary vascular resistance;
- well-developed pulmonary arteries;
- good ventricular systolic function;
- good ventricular relaxation;
- minimal atrioventricular valve regurgitation;
- unobstructed systemic venous pathways;
- and no significant obstruction to systemic ventricular outflow.
Because there is no pulmonary ventricle, there is very little reserve for overcoming pulmonary resistance.
The entire circulation therefore depends upon maintaining a favourable pressure gradient through the lungs.
Anaesthetic Implications of Fontan Physiology
Anaesthetic management of a Fontan patient requires understanding that pulmonary blood flow depends heavily upon venous pressure and pulmonary vascular resistance.
Factors that can reduce pulmonary blood flow include:
- hypovolaemia;
- high intrathoracic pressure;
- excessive positive-pressure ventilation;
- hypoxia;
- hypercarbia;
- acidosis;
- and increased PVR.
Venous return is effectively the driving pressure for pulmonary circulation.
Maintaining preload is therefore particularly important.
Spontaneous ventilation can sometimes be advantageous because negative intrathoracic pressure assists venous return and pulmonary blood flow.
Positive-pressure ventilation may reduce that gradient if not carefully managed.
Norwood Physiology Versus Fontan Physiology
It is useful to recognise how profoundly the circulation changes across the staged pathway.
After Norwood
The single ventricle supplies both systemic and pulmonary circulations.
Blood is substantially mixed.
The circulation remains effectively parallel.
After Glenn
The SVC supplies part of pulmonary blood flow directly.
The single ventricle still receives IVC blood and pulmonary venous blood.
The circulation is partially separated.
After Fontan
Both SVC and IVC blood travel directly towards the lungs.
The single ventricle predominantly handles systemic output.
The circulation has been converted as close as possible to a series arrangement without creating a second ventricle.
Palliative, Not Corrective
This distinction is essential.
Norwood, Glenn and Fontan procedures do not cure the original congenital abnormality.
They do not create a normal heart.
They reconstruct the circulation to allow one functioning ventricle to sustain life.
The term staged palliation is therefore more accurate than “corrective surgery.”
Modern survival has improved greatly, and increasing numbers of people with Fontan circulation are now reaching adulthood. However, the physiology remains fundamentally abnormal and requires lifelong specialist surveillance. (PubMed Central (PMC))
Long-Term Fontan Complications
The absence of a pulmonary ventricle produces chronically elevated systemic venous pressures and relatively limited ventricular preload.
Over many years this can affect multiple organ systems.
Potential complications include:
- ventricular dysfunction;
- atrioventricular valve regurgitation;
- atrial and ventricular arrhythmias;
- Fontan pathway obstruction;
- thromboembolism;
- protein-losing enteropathy;
- plastic bronchitis;
- lymphatic abnormalities;
- reduced exercise capacity;
- renal dysfunction;
- and liver disease.
Fontan-associated liver disease — FALD has become an increasingly important aspect of long-term care as the Fontan population ages. (PubMed Central (PMC))
A Fontan circulation therefore involves the whole body, not simply the heart.
Protein-Losing Enteropathy
One particularly important Fontan complication is protein-losing enteropathy — PLE.
Elevated venous and lymphatic pressures can contribute to loss of protein through the gastrointestinal tract.
Patients may develop:
- low serum albumin;
- oedema;
- ascites;
- diarrhoea;
- immune abnormalities;
- and nutritional difficulties.
It can be a serious manifestation of Fontan circulatory failure.
Plastic Bronchitis
Another unusual but characteristic complication is plastic bronchitis.
Abnormal lymphatic flow can result in proteinaceous material entering the airways and forming branching casts.
These casts can obstruct bronchi and cause severe respiratory symptoms.
Modern understanding of Fontan complications increasingly recognises the importance of the lymphatic circulation.
Fontan-Associated Liver Disease
Chronically elevated systemic venous pressure is transmitted to the hepatic circulation.
Over time this can cause:
- hepatic congestion;
- fibrosis;
- cirrhosis;
- portal hypertension;
- and an increased risk of hepatic malignancy.
The liver therefore forms an important part of lifelong Fontan surveillance. (PubMed Central (PMC))
Exercise and the Fontan Circulation
During exercise, a normal right ventricle increases pulmonary blood flow.
A Fontan patient does not have that pump.
Pulmonary blood flow can therefore become a limiting factor during exertion.
Many patients have reduced peak exercise capacity even when the circulation is functioning relatively well.
This does not mean exercise should automatically be avoided.
Appropriate physical activity and specialist advice can be important components of long-term care.
When the Fontan Fails
Fontan failure is not one single disease.
It may involve:
- ventricular failure;
- valve dysfunction;
- rising pulmonary vascular resistance;
- pathway obstruction;
- arrhythmia;
- lymphatic failure;
- liver disease;
- or combinations of these problems.
Interventions can include catheter procedures, arrhythmia treatment, valve surgery, Fontan revision, mechanical circulatory support in selected circumstances and ultimately transplantation.
For some patients, heart transplantation becomes the final option when Fontan physiology can no longer sustain adequate circulation.
Single-Ventricle Palliation in One Sentence
Single-ventricle palliation progressively removes the pulmonary circulation from the workload of the single functional ventricle, ultimately directing systemic venous blood passively through the lungs while reserving the ventricle principally for systemic output.
The sequence can be remembered simply:
Norwood — create survivable systemic and pulmonary blood flow.
Glenn — send upper-body venous blood directly to the lungs.
Fontan — send essentially all systemic venous blood directly to the lungs.
Key Points
Single-ventricle physiology describes congenital hearts in which two effective ventricular circulations cannot be established.
It includes several different congenital defects rather than one single diagnosis.
The operations are palliative rather than anatomically corrective.
The Norwood is a Stage I strategy principally used when systemic outflow is inadequate, particularly in HLHS. Not every single-ventricle patient requires a Norwood. (PubMed Central (PMC))
The Bidirectional Glenn directs SVC blood directly to both pulmonary arteries and reduces ventricular volume loading.
The Fontan completes the cavopulmonary connection by directing IVC blood to the pulmonary circulation.
After Fontan completion there is no dedicated pulmonary ventricle.
Fontan pulmonary blood flow depends on low pulmonary vascular resistance and adequate systemic venous pressure.
Modern patients can survive for decades with Fontan physiology, but lifelong congenital cardiac and multisystem follow-up is essential. (PubMed Central (PMC))
Glossary of Terms
Atriopulmonary Fontan
An early form of Fontan operation connecting the right atrium to the pulmonary artery.
It has largely been replaced because of long-term complications including atrial dilation and arrhythmias.
Bidirectional Glenn
A superior cavopulmonary connection in which the SVC is connected to the pulmonary artery while maintaining continuity between the right and left pulmonary arteries.
Blalock–Thomas–Taussig Shunt — BTT Shunt
A systemic-to-pulmonary arterial shunt used to provide pulmonary blood flow.
A modified BTT shunt can be used as part of Stage I Norwood palliation.
Cavopulmonary Connection
A direct connection between the systemic venous circulation and pulmonary arteries.
The Glenn and Fontan are forms of cavopulmonary connection.
Extracardiac Conduit
A tube graft placed outside the heart connecting the IVC to the pulmonary arteries during one form of Fontan completion.
Fenestration
A deliberately created small communication between the Fontan pathway and atrium, allowing decompression of excessive venous pressure at the cost of some systemic desaturation.
Fontan Circulation
A circulation in which systemic venous blood travels directly to the pulmonary arteries without being pumped by a subpulmonary ventricle.
Fontan-Associated Liver Disease — FALD
Progressive hepatic abnormalities related to chronically elevated systemic venous pressure and altered circulation following Fontan palliation.
Glenn Procedure
A surgical connection directing superior vena caval blood into the pulmonary artery.
Modern Stage II palliation usually uses the bidirectional Glenn.
Hemi-Fontan
An alternative Stage II cavopulmonary operation that directs SVC blood towards the pulmonary arteries and facilitates later Fontan completion.
Hypoplastic Left Heart Syndrome — HLHS
A spectrum of severe congenital abnormalities in which the structures of the left heart are too poorly developed to support systemic circulation effectively.
Inferior Vena Cava — IVC
The major vein returning deoxygenated blood from the lower body to the heart.
Lateral Tunnel Fontan
A Fontan technique using an intra-atrial baffle to direct IVC blood towards the pulmonary arteries.
Neoaorta
A surgically constructed systemic outflow vessel.
During the Norwood operation, the pulmonary root contributes to the reconstructed systemic outflow.
Norwood Procedure
A Stage I palliative operation used principally in single-ventricle conditions with inadequate systemic outflow.
It establishes systemic outflow, unrestricted atrial mixing and controlled pulmonary blood flow.
Palliation
Treatment designed to create sustainable physiology without restoring completely normal anatomy.
Norwood, Glenn and Fontan surgery constitute staged palliation.
Plastic Bronchitis
A complication in which abnormal lymphatic material forms branching casts within the airways.
It can occur in patients with Fontan circulation.
Protein-Losing Enteropathy — PLE
Loss of protein through the gastrointestinal tract, sometimes associated with elevated venous and lymphatic pressures in Fontan physiology.
Pulmonary Vascular Resistance — PVR
Resistance to blood flow through the pulmonary circulation.
Low PVR is particularly important to successful Glenn and Fontan physiology.
Sano Conduit
A conduit between the right ventricle and pulmonary artery used as one method of providing pulmonary blood flow during Norwood palliation.
Single-Ventricle Physiology
A congenital circulation in which only one ventricle can effectively support the circulation, or in which a two-ventricle repair cannot safely be established.
Superior Vena Cava — SVC
The major vein returning systemic venous blood from the head, neck and upper body.
Total Cavopulmonary Connection — TCPC
A circulation in which both superior and inferior vena caval blood is directed to the pulmonary arteries.
Modern Fontan circulation is based on this principle.
